1. Field of the Invention
The present invention relates to a damping force control type hydraulic shock absorber attached to a suspension system of a vehicle, for example, an automobile.
2. Description of the Related Art
Hydraulic shock absorbers attached to suspension systems of automobiles or other vehicles include damping force control type hydraulic shock absorbers which are designed so that the level of damping force can be properly controlled in accordance with the road surface conditions, vehicle running conditions, etc., with a view to improving the ride quality and the steering stability.
In general, this type of hydraulic shock absorber includes a cylinder having a hydraulic fluid sealed therein, and a piston which has a piston rod connected thereto and which is slidably fitted in the cylinder to define therein two cylinder chambers. The two cylinder chambers are communicated with each other by a main hydraulic fluid passage and a bypass passage. The main hydraulic fluid passage is provided with a damping force generating mechanism (including an orifice, a disk valve, etc.) for generating a relatively large damping force. The bypass passage is provided with a damping force generating mechanism (including an orifice, a disk valve, etc.) for generating a relatively small damping force, and with a damping force control valve for opening and closing the bypass passage.
With the above-described arrangement, when the damping force control valve is opened, the hydraulic fluid in the cylinder is induced to flow mainly through the bypass passage by the sliding movement of the piston caused by the extension and contraction of the piston rod, thereby generating a relatively small damping force. Accordingly, the damping force characteristics are "soft". When the damping force control valve is closed, the hydraulic fluid in the cylinder is induced to flow only through the main hydraulic fluid passage by the sliding movement of the piston caused by the extension and contraction of the piston rod, thereby generating a relatively large damping force. Accordingly, the damping force characteristics are "hard". Thus, the damping force characteristics can be changed by opening and closing the damping force control valve.
Incidentally, when a damping force control type hydraulic shock absorber of the type described above is designed to have valve damping force characteristics in which the damping force linearly changes in accordance with the piston speed for both the "hard" and "soft" characteristics, an arrangement as described, for example, in Japanese Patent Application Public Disclosure (KOKAI) No. 04-54339 has heretofore been employed. That is, a piston, which is fitted in a cylinder, and a piston rod, which is connected to the piston, are provided with a main hydraulic fluid passage, a bypass passage, and a damping force control valve for opening and closing the bypass passage, and the main hydraulic fluid passage and the bypass passage are provided with respective valve mechanisms. In addition, a control rod for actuating the damping force control valve extends through the piston rod and projects outwardly from the outer end of the piston rod, so that the damping force characteristics can be changed over from one to another by actuating the control rod with an actuator or the like from the outside of the hydraulic shock absorber.
However, the conventional damping force control type hydraulic shock absorber, in which a piston assembly consisting of a piston and elements connected thereto is provided with a bypass passage and a damping force control valve, suffers from problems as described below. That is, since the piston assembly is provided with a bypass passage, and the bypass passage is provided with a control valve or other element for controlling the damping force, the size of the piston assembly increases, and the extension and contraction stroke of the piston rod inevitably shortens.
In addition, the actuator for operating the control rod for the damping force control valve is usually attached to the distal end of the piston rod, which is connected to a member provided on the vehicle body. Therefore, a piston rod mounting member (e.g., a strut mount) installed on the vehicle body must be provided with a space for mounting the actuator, which deteriorates the space efficiency on the vehicle body. Accordingly, the conventional arrangement goes against the demand for the engine hood to lower at the piston rod mounting part on the front wheel side of an automobile. In particular, a suspension system in which the damping force characteristics of a hydraulic shock absorber are controlled in real time during running (i.e., what is called a semiactive suspension system) needs a large-sized actuator for rapidly changing the damping force characteristics. Therefore, the deterioration of the space efficiency becomes a serious problem.